The NMR phenomena is a magnetic field sensitive response of nuclear magnetic moments (or "spins") to transfer of energy from an external energy source to the nuclear spin system. The phenomena is applied in many different forms, but largely the underlying physical operation may be regarded as a measurement of magnetic field in term of an RF frequency. Broadly speaking, the phenomena is exploited for applications in the field of analytical chemistry and separately for acquiring representations of spatial distributions of compositions exhibiting certain chemical characteristics. An example of the latter is apparatus for realizing an image of a sample. These are not exclusive purposes: spatial distribution maps, e.g., medical imaging, also include aspects of analytic chemical discriminator, and applications in analytic chemistry may utilize spatial discrimination. The phenomena demands a locally defined magnetic environment whether such magnetic environment includes a known field which is static and uniform or transient and non-uniform.
In order to assure control of the magnetic environment, the sample space is carefully shimmed, that is to say, compensated, to reduce undesired magnetic field components. This is accomplished with static shim coils designed to controllably impost a magnetic field of selected direction and/or symmetry to cancel a residual magnetic field in the sensitive volume of the instrument. These shim coils are often disposed in the room temperature peripheral space of the bore of a superconducting magnet.
The achievement of a high degree of uniformity is essential for the magnetic environment of a sample to be studied via NMR phenomena. Controlled non-uniformity of the same magnetic environment is required for a number of measurement techniques. These techniques demand precise control in spatial dependence. Transient gradient fields add further requirements for both spatial and time dependence of the desired magnetic field. Examples of such apparatus are in the spatial encoding of information in NMR signals; selective excitation and/or detection of specific quantum transitions; and, observation or exploitation of various diffusion related phenomena.
Desired magnetic field gradients are obtained with coils designed to furnish the required directional and spatial dependence in cooperation with respective current sources controllable to yield the desired amplitude, duration and perhaps functional time dependence. These coils are disposed at the periphery of the bore of the superconducting magnetic in close proximity to the shim coils. As a consequence of the close proximity of (pulsed) gradient coils and (steady state) shim coils, there is induced in (certain) shim coils a current component due to application of the transient condition to the gradient coil. This parasitic effect maximal for shim coils or gradient coils which generate roughly parallel (or anti-parallel) field components and minimal in the case where the mutual orientation of fields produced by the shim coil and the gradient coil are orthogonal.
The problem is exacerbated in narrow bore instruments because the shim and gradient coils are necessarily disposed quite close together. In very wide bore equipment, such as medical imagers, there if frequently no provision for shim coils, or the available space permits some displacement of shim coils and gradients coils or provision is made for shielding means between them to attenuate the coupling therebetween.
It is known to employ a corresponding gradient amplifier-gradient coil combination to furnish the shim field component during a transient, leaving the shim coil disconnected from its power supply and directing the shim power supply output to the gradient amplifier. A typical gradient amplifier is rated to deliver of the order of 10.sup.2 amps for the transient gradient excitation. The shim field components consumes of the order of 10.sup.-3 amps in the steady state. The steady state performance at the lower output current level is affected by amplifier noise. This is qualified by the observation that shimming via gradient coils as above described may secure homogeneity of the order of 2 to 5 Hertz (as measured by the observed width of a sharp resonant line) whereas the shimming operation via static shim coils may yield an observed line width of the order of 0.2 to 0.5 Hertz.
The nature of the invention between these (undesirably) coupled coils may be quite complex because the respective circuits ordinarily exhibit diverse time constant and in general one observes damped oscillations ("ringing") in the response of the shim coil.